The air oxidation of cyclohexane is an important process for the production of caprolactam and adipic acid, which are used in the manufacture of synthetic products, such as nylon. The oxidation of cyclohexane by air produces a reaction product comprising cyclohexanol (A), cyclohexanone (K) cyclohexylhydroperoxide (CHHP) and small amounts of by-products. Cyclohexanone (K) and cyclohexanol (A) are the main product of the overall process and the mixture is commonly known as KA oil. Several patents, herein incorporated by reference, such as U.S. Pat. Nos. 3,530,185, 3,987,100, 5,780,683, 6,888,034 and 6,703,529 teach the preparation of a mixture containing cyclohexanol, cyclohexanone and cyclohexylhydroperoxide by the air oxidation of cyclohexane.
It is well known that CHHP in a mixture containing cyclohexanol, cyclohexanone, other products of the air oxidation reactions will react to form KA oil. However, this process does not result in a high yield of KA oil and other waste materials are formed. It has been found that the highest yields of KA oil can be achieved when the oxidation of cylcohexane is performed under conditions that result in a greater amount of CHHP and the CHHP is then treated by hydrogenation in a separate process to cyclohexanone (K) and cyclohexanol (A) to give an increased overall yield of KA oil. For example, the preparation of cyclohexanol and cyclohexanone from cyclohexylhydroperoxide by hydrogenation has been in described in U.S. Pat. Nos. 3,694,511 and 3,927,108, herein incorporated by reference.
The air oxidation reaction is generally conducted at temperatures from about 130° C. to about 200° C. Various types of reactors can be implemented for commercial use and these include single autoclaves, multiple autoclaves in series, horizontal single reactors with multiple compartments, and multistage column reactors. Air is generally used as the primary source of oxygen. Any unreacted oxygen (along with the nitrogen present in the air) leaves the reactor or reactors as a gaseous effluent or off-gas. The off-gas also contains vaporized cyclohexane and other compounds. The amount of unreacted oxygen in the off-gas is commonly referred to as “oxygen leakage.” The vaporized cyclohexane and other products in the off-gas are condensed and recovered, and the off-gases leave the system, usually to an abatement system. The oxidation products that are produced from the oxidation reaction are recovered from the liquid effluent from the reactor or reactors, and the unreacted cyclohexane is recycled.
It has been observed that at lower oxygen leakage levels from a reactor, the higher the formation of undesirable byproducts and hence the lower the yield to desirable oxidation products. In the oxidation of cyclohexane, the yield of cyclohexanone, cyclohexanol and CHHP, can be optimized by operating at high oxygen leakage (i.e. concentration of unreacted oxygen in the mixture of cyclohexane free oxygen, nitrogen and other gases and vapors). Unfortunately, at oxygen leakage concentration in excess of 8 vol %, unsafe flammable mixtures can form in the effluent gas stream. Therefore, as a margin of safety the oxygen leakage is usually kept below 4 vol %. Higher oxygen leakage also means that the air being fed to the reactor(s) is not being fully utilized. In other words, the process will requires more air, which leads to increased compression cost. In addition, an increased volume of off-gas causes increased cost for off-gas treatment.
U.S. Pat. No. 3,957,876 teaches a method to reduce oxygen leakage from a cyclohexane oxidation process through the use of oxygen clean up zones. The oxygen clean up zones allows additional consumption of oxygen by reacting it with cyclohexane and thus produces an off-gas that contains oxygen of adequately low concentration so that an explosion hazard can be avoided.
One disadvantage of the prior art is that it is difficult to maintain a desired range of oxygen leakage leaving the reactor, while maintaining the desired yield from the cyclohexane oxidation reaction. The examples in U.S. Pat. No. 3,957,876 teach an oxygen leakage ranging from 2% to 10% by volume.
It has been found that the temperature profile in the reactor is critical to acquiring a desired yield from the oxidation reaction and maintaining the level of oxygen leakage. The temperatures in the reaction section of the column must be maintained sufficiently high to sustain the oxidation reaction. However, excessively high temperatures are detrimental to yield because they will lead to an increased rate of oxidation of the KA and CHHP to undesired byproducts.
Therefore, there is a need for an improved process for the air oxidation of cyclohexane, wherein the temperature profile in the oxidation reactor is maintained in order to maximize the yield of desired oxidation products and control the oxygen leakage in the reactor off-gas.